This invention relates generally to radar jamming and more particularly to a method of operating a radar system which will function effectively in the presence of jamming signals.
An enemy aircraft radiating high power density noise jamming across the entire operating frequency band of a search or tracking radar represents a most formidable electronic countermeasures threat, and virtually no radars exist which can track the skin echo from small aircraft under these conditions. However, two jamming elimination methods have been investigated which are each capable of 20 db or more reduction in the jamming power: monopulse cancellation and polarization cancellation.
The former uses an appropriately amplified and phase shifted jamming signal from a monopulse .DELTA.-beam to cancel the same jamming waveform from the .SIGMA.-beam, thus revealing the skin return. The latter method is similar, but uses, for example, the jamming waveform from a horizontally polarized feed to cancel the jamming from the vertically polarized feed.
Both methods use a "correlation and cancellation" feedback loop to derive and effect the required amplitude and phase adjustment necessary for cancellation. However, the significant item upon which a successful cancellation method depends is that the main and auxiliary channel signals must have a different vector combination of the jamming waveform and the target echo when it occurs. For purposes of illustration, if the main and auxiliary channel signals are s(t)+j(t) and .alpha.[s(t)+.beta.j(t)]respectively, where a and B are arbitrary complex constants; then a successful cancellation method i.e., jamming cancelled, signal not cancelled, can result as long as .beta..noteq.1.
In the case of polarization cancellation, the required difference arises from a difference in the polarization of the target echo and the incident jamming.